In the last years, mobile devices have become ubiquitous, applications for these mobile devices have flooded the market, and clients' expectations for omnipresent high quality service have challenged the network service providers. Among other things, users expect services to be delivered with high quality, and some services require high quality in the radio communications in order to be perceived by users as being delivered satisfactorily. Unfortunately, in a radio environment, there will be situations where some, e.g., data packets, are not received properly by a user's equipment and, in such cases, it may be necessary to retransmit those data packets in order to provide an appropriate level of service.
The telecommunications industry has used the Automatic Repeat reQuest (ARQ) layer 2 protocol for many years as a retransmission mechanism to ensure that data is sent reliably from one node to another. More recently, certain standards, such as the Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) standard, have taken advantage of a Hybrid ARQ (HARQ) process to counteract errors. See FIG. 1 and FIG. 2 for the illustrations of the Layer 2 architecture used in LTE.
FIG. 1 shows the Layer 2 architecture for downlink (DL) 2 used in LTE and includes the Packet Data Convergence Protocol (PDCP) sublayer 4, the Radio Link Control (RLC) sublayer 6 and the Medium Access Control (MAC) sublayer 8. The PDCP layer 4 includes a plurality of Robust Header Compression (ROHC) functions 10 each of which has its own security function 12 which can perform, for example, ciphering. The RLC sublayer 6 can perform error correction through ARQ and re-segmentation as shown in function blocks 14. The MAC sublayer 8 can include a scheduling and priority handling function 16, perform multiplexing/demultiplexing of data as shown in blocks 18 and 20. Additionally, error correction through HARQ 22 can also be performed. The Layer 2 architecture for DL 2 also includes control channels such as a Broadcast Control Channel (BCCH) 24, a Common Control Channel (CCCH) 26 and a Paging Control Channel (PCCH) 28.
FIG. 2 shows the Layer 2 architecture for uplink (UL) 30 used in LTE which is similar to the architecture described above with respect to FIG. 1, i.e., FIG. 2 includes some of the same type of functions designed for use in the UL. For more information regarding these Layer 2 architectures the interested reader is directed to 3GPP TS 36.300 V11.3.0 (2012-09).
HARQ involves an encoded forward link for error correction and detection, and a feedback link for possible retransmission. At the transmitter, parity bits are added to a data block which is to be transmitted, the parity bits serving to facilitate detection and correction of errors. In case the receiver is not able to correct these errors, the data block is transmitted again. For each received data block the receiver either sends a positive acknowledgment (“ACK”) (data block is received or decoded successfully) or a negative acknowledgement (“NACK”) (data block is undecodable). The transmitter responds to a NACK by re-transmitting the information.
HARQ is a stop-and-wait protocol. Being a stop-and-wait protocol, (re)transmissions are restricted to occur at known time instants, in between which the sender stops and waits for ACK/NACK feedback from the receiver. As used herein, “feedback”, and particularly HARQ feedback, includes both feedback of a positive acknowledgement (“ACK”) and feedback of negative acknowledgement (“NACK”). Thus, subsequent transmission of new data can take place only after waiting to receive ACK/NACK from the receiving entity. In case an ACK is received a new transmission occurs, otherwise a retransmission occurs. This scheme can be improved by using multiple channels for supporting HARQ service. The HARQ receiver must transmit either ACK or NACK, still of course there is the possibility that the sender detects neither. This is referred to as detection of a Discontinuous Transmission (DTX). There are two possible reasons for a DTX detection to occur, either the data was lost or the forward transmission was lost and not detected by the receiver.
When the HARQ transmitter has reached the maximum number of retransmissions for a transport block without getting an ACK, the HARQ transmitter will stop transmitting and let a higher layer ARQ take over, if any such higher layer ARQ exists. Examples of such higher layers that engage ARQ are the Radio Link Control (RLC), 3GPP TS 36.322 V11.0.0 (2012-09), and the Transmission Control Protocol (TCP), RFC 2581 (1999-04).
The standards document 3GPP TS 36.300, Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2, version 11.3.0, sub-clause 9.1 explains some of the principles of LTE HARQ used by 3GPP. For LTE HARQ, there are several channels of interest, including the Physical Uplink Control Channel (PUCCH); the Physical Uplink Shared Channel (PUSCH); the Physical Downlink Control Channel (PDCCH); and, the Physical Hybrid ARQ Indicator Channel (PHICH).
For the UL, i.e., for transmissions in the UL from a user equipment (UE) to the base station, the LTE HARQ is a Synchronous HARQ. There is a maximum number of retransmissions configured per UE (as opposed to per radio bearer) with RRC parametermaxHARQ-Tx. Downlink ACK/NACKs in response to uplink (re)transmissions are sent on the PHICH. The radio channel may act in an obstructive way such that ACK can be received as NACK and vice versa. The PHICH can also be totally obscured in such a way that the peer receiver detects DTX.
In general, HARQ operation on the UL is governed by the following basic principles. A first UL HARQ operation principle is that, regardless of the content of the received HARQ feedback (ACK, NACK or DTX), when a PDCCH for the UE is correctly received, the UE follows what the PDCCH asks the UE to do, e.g., the UE performs a transmission or a retransmission (referred to as adaptive retransmission). A second UL HARQ operation principle is that, when no PDCCH addressed to the Cell Radio Network Temporary Identifier (C-RNTI) of the UE is detected, the HARQ feedback on PHICH dictates how the UE performs retransmissions. If the HARQ feedback to the UE is either NACK or DTX, the UE performs a non-adaptive retransmission, i.e., a retransmission on the same uplink resource as previously used by the same process. If the HARQ feedback to the UE is an ACK, the UE does not perform any UL (re)transmission but still keeps the data in a buffer known as the HARQ buffer. The PDCCH can then be used to perform a retransmission, but a non-adaptive retransmission cannot follow.
3GPP TS 36.321, Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification, Medium Access Control (MAC) protocol specification, version 11.0.0, sub-clause 5.4.2, specifies the HARQ transmitter operation of the E-UTRAN UE as a set of sub layer procedures of layer 2. According to this standards specification and sub-clause, an identified HARQ process of the UE (HARQ transmitter over the Uplink Shared Channel [UL-SCH]) is instructed to generate an adaptive retransmission if an uplink grant is detected, and otherwise to generate a non-adaptive retransmission.
3GPP TS 36.321 specifies in different sub-clauses three different conditions that each by itself will clear any pending HARQ retransmission in a UE. Each is associated to an explicit flush of the HARQ buffer, which effectively clears any pending retransmissions. More specifically:
Sub-clause 5.2. The UE is not allowed to transmit when it is not synchronized to the reception timing used by the base station; the UE will flush the HARQ buffer at expiry of the timer supervising time alignment (timeAlignmentTimer);
Sub-clause 5.4.2.2. The HARQ persistency are brought to an end when the maximum number of transmissions is reached; the UE will flush the HARQ buffer when the variable CURRENT_TX_NB=maximum number of transmissions−1; and
Sub-clause 5.9. The MAC layer in UE will reset if requested by higher layers; all timers are stopped, all variables reset and all buffers are flushed.
In addition, 3GPP TS 36.321 specifies in sub-clause 5.4.2.2 one condition that will postpone any pending HARQ retransmission. It is summarized in the following note. NOTE: When receiving a HARQ ACK alone, the UE keeps the data in the HARQ buffer. Such a postponed HARQ retransmission is always an adaptive retransmission, i.e., its occurrence is explicitly controlled by an eNodeB and the UE must conclude indication of an uplink grant for the associated HARQ process.
There are certain conventional scenarios which benefit from the postponing of HARQ retransmissions, e.g., when some parallel procedures occur. For example, Change Request (CR) 5 on 36.321 in 3GPP R2-084875 made it clear that measurement gaps have high priority. The UE will not perform UL transmissions based on an UL grant in such a gap. This is one scenario where an eNodeB may not yet have successfully received some UL transmission but decides to postpone all associated HARQ retransmissions in the UE. This can be done sending HARQ ACK before the measurement gap.
The same CR also made it clear that random access has higher priority, even higher priority than measurement gaps. This is another scenario where an eNodeB may not yet have successfully received some UL transmission but decides to postpone all associated HARQ retransmission in the UE, e.g., the network may want to give priority to a Msg3 over HARQ retransmission. A Msg3 is a message transmitted on the UL-SCH containing a C-RNTI MAC Control Element (CE) or a Common Control Channel (CCH) Service Data Unit (SDU), submitted from an upper layer and associated with the UE contention resolution identity, as part of a random access procedure.
There are other scenarios which benefit from the postponement of HARQ retransmissions in the UL, e.g., when an eNodeB experiences congestion on PDCCH, PDSCH or PUSCH. In a high network load scenario, the processing capability decreases as traffic grows closer to system capacity limits. In this case, the network elements will benefit from the possibility to ease the load by postponing some retransmissions.
NACK-to-ACK errors are another reason why it is useful to be able to perform HARQ retransmissions instead of stopping them after HARQ ACK. The radio quality gets worse as UE moves close to the cell border. Not only does the occurrence of NACK increase but also the probability of NACK-to-ACK error. In the case of NACK-to-ACK error, the UE does not perform non-adaptive retransmission even though it is instructed to do so. The network can detect missing UL retransmission and trigger adaptive retransmission one HARQ RTT later. Thus, there are good reasons why ACK alone shall not always unconditionally flush the HARQ buffer.
Turning now to the topic of discontinuous reception (DRX), the “always-on” type of behavior that is arriving with smartphones adds much strain on the battery economy in the UE. There are different methods in LTE to limit the power consumption in the UE, one such method being DRX.
DRX can be applied both in RRC_IDLE and in RRC_CONNECTED states. The principles are the same. The description hereafter applies to DRX during RRC_CONNECTED which is the relevant state for the abovementioned “always-on” type of behavior.
DRX is a method to reduce battery consumption in the UE by allowing the UE to stop monitoring the PDCCH, i.e., it can turn off the receiver during short and even lengthy times and just discontinuously listen during shorter so called on-duration phases, the occurrence of which are known to both sides of the protocol. The time periods where the UE is allowed to turn off the receiver are configured by the network and acknowledged by the UE. The reoccurring periods of the on-duration phase is illustrated in FIG. 3.
For example, as shown in FIG. 3, there is a DRX Cycle 32 which includes an On Duration Time 34, which is when the UE monitors the PDCCH, and an opportunity for DRX 36, i.e., a time when the UE can turn off its receiver.
FIG. 4 illustrates the LTE state model and the denotations used in this description. For example, FIG. 4 shows the transition paths between RRC_IDLE state 38 and RRC_CONNECTED state 40 and furthermore the DRX transitions between sub-states (Continuous/Active sub-state 42, Short DRX sub-state 44 and Long DRX sub-state 46) while in the RRC_CONNECTED state 40. FIG. 4 also shows the conventional view of energy and latency associated to the states and sub-states of LTE which is that the more power that is used by the device modem, the faster it will respond to data communication. Staying in the Continuous/Active sub-state 42 is better for having a more immediate connection than staying in the lower DRX sub-states 44 and 46. Staying in the Short DRX sub-state 44 in turn provides better responsiveness than staying in the Long DRX sub-state 46. Staying in the RRC_CONNECTED state 40 in turn results in faster reaction (better latency) than staying in the RRC_IDLE state 38. FIG. 4 also illustrates how the energy consumed by the device will change and suggests that the battery lifetime is shortest when staying all the time in Continuous/Active sub-state 42 of the RRC_CONNECTED state 40.
DRX involves the use of timers to supervise active reception time. The 3GPP TS 36.321, chapter 3 and chapter 5, sub-clause 5.7 specifies the drx-InactivityTimer (denoted T1 in FIG. 4) to be the number of consecutive downlink subframe(s) during which the UE shall monitor the PDCCH after successfully decoding a PDCCH indicating an initial UL or DL user data transmission for this UE.
3GPP TS 36.321 also specifies drxShortCycleTimer (denoted T2 in FIG. 4) to be the number of consecutive subframe(s) the UE shall follow the short DRX cycle after the drx-InactivityTimer has expired. FIG. 5 is an illustration of the sub-state transitions following upon inactivity.
As shown in FIG. 5, the UE will start the drx-InactivityTimer 48 each time it terminates and decodes a PDCCH indicating new transmission. It will then continue to monitor PDCCH as long as the timer is running, i.e., the drx-InactivityTimer 48 will keep the UE from falling asleep. After the drx-InactivityTimer 48 has expired the drxShortCycleTimer 50 is started to supervise a switch to the next lower DRX sub-state using Long DRX cycles 52. The Short DRX cycles 54 are typically much shorter than the Long DRX cycles 52.
The concept of “Active Time” is also used in DRX. 3GPP TS 36.321, version 11.0.0, chapter 5, subclause 5.7, defines the Active Time as the aggregated phases while either:                on DurationTimer or drx-InactivityTimer or drx-RetransmissionTimer (used to supervise any DL retransmission that UE expects) or mac-ContentionResolutionTimer (used to supervise the completion of random access) is running; or        a Scheduling Request for UL transmission is sent on PUCCH and is pending (no grant for UL transmission has yet been received); or        an uplink grant for a pending HARQ retransmission can occur and there is data in the corresponding HARQ buffer; or        a PDCCH indicating a new transmission addressed to the C-RNTI of the UE has not been received after successful reception of a Random Access Response for the preamble not selected by the UE.        
In particular, 3GPP TS 36.321, subclause 5.7, mandates the UE to start or restart drx-InactivityTimer if the PDCCH indicates a new transmission (DL or UL). The on DurationTimer mentioned above is another standardized timer that supervises the duration of the on-duration phase.
It should be noted that the third requirement above intends to make sure that the UE is monitoring PDCCH for adaptive retransmission grants when the HARQ retransmission has been suspended by HARQ ACK or there has been NACK-to-ACK error.
From the foregoing discussion, it will be apparent that in current systems the eNodeB does not have any immediate means to completely stop retransmissions, but can only halt non-adaptive retransmissions, e.g., ACK on PHICH always results in suspension. One problem with systems and methods which employ such an unconditional ACK suspension is a waste of battery power. Accordingly, it would be desirable to provide devices, systems, nodes and methods that would alleviate the impact of these problems.